(a) Field of the Invention
The present invention relates to a novel compound having a photoreactive functional group, a photoreactive polymer, and an alignment film comprising the same. More particularly, the present invention relates to a photoreactive polymer that shows excellent alignment stability and thermal stability together with excellent liquid crystal alignment, thereby being desirably used in an alignment film of a liquid crystal display device, a compound having a photoreactive functional group that is used as a monomer for the preparation of the photoreactive polymer, and an alignment film.
(b) Description of the Related Art
In recent years, as a liquid crystal display has become bigger, its application has been expanded from personal mobile phone or notebook computer to wall-mounted television, and thus it is required to ensure the high definition, the high quality, and the wide viewing angle in respects to the liquid crystal display. In particular, since a thin film transistor liquid crystal display (TFT-LCD) driven by a thin film transistor independently drives each of pixels, a response rate of the liquid crystal is very high, and thus a high-quality dynamic image can be realized. Accordingly, the application range thereof is expanded. In order to use liquid crystals as an optical switch in the TFT-LCD, liquid crystals needs to be initially oriented in a predetermined direction on a thin film transistor, which is disposed in the most inner portion of a display cell. For this purpose, a liquid crystal alignment film is used.
To achieve the liquid crystal alignment, a rubbing process is applied, in which a heat resistant polymer such as polyimide is applied on a′ transparent glass to form a polymer alignment film and the alignment film is rubbed by rotating a rotation roller wound with a rubbing cloth made of nylon, rayon or the like at a high speed.
However, since the rubbing process may cause mechanical scratches or high static electricity on the surface of the liquid crystal alignment material during the rubbing process, a thin film transistor may be destroyed. In addition, a defect occurs due to fine fibers generated from the rubbing cloth, which hinders the improvement in production yield.
In order to overcome the problem of the rubbing process for the productivity improvement, a novel liquid crystal alignment process is designed, which is a liquid crystal alignment by using UV, namely, light (hereinafter, referred to as “optical alignment”).
The optical alignment refers to a mechanism, in which a photosensitive group connected to the photoreactive polymer generates a photoreaction due to linearly polarized UV, and in this procedure, a main chain of the polymer is unidirectionally aligned, thereby forming a photopolymerizable liquid crystal alignment film in which the liquid crystals are aligned.
A representative example thereof is an optical alignment by photopolymerization, which is announced by M. Schadt, et al. (Jpn. J. Appl. Phys., Vol 31., 1992, 2155), Dae S. Kang, et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000). In these patent documents and papers, polycinnamate-based polymers such as PVCN (poly(vinyl cinnamate)) and PVMC (poly(vinyl methoxycinnamate)) are generally used as the optical alignment polymers. In the case of performing the optical alignment, the cycloaddition reaction [2+2] of the double bond [2+2] of cinnamate by UV forms cyclobutane, and thus an anisotropic property is formed to unidirectionally align liquid crystal molecules, leading to the alignment of the liquid crystals.
In addition, Japanese Unexamined Patent Application Publication No. 11-1811127 discloses a polymer having a main chain such as acrylate and methacrylate and a side chain containing a photosensitive group such as a cinnamate group, and an alignment film comprising the same. Korean Patent Publication No. 2002-00068195 also discloses use of alignment film produced by using a polymethacryl-based polmer.
However, the aforementioned optical alignment polymers are disadvantageous in that thermal stability of the polymer main chain is reduced to deteriorate alignment stability or thermal stability of the alignment film, or the liquid crystal alignment becomes insufficient. For example, a polymer having an acryl-based main chain greatly reduces the stability of the alignment film due to the low thermal stability, and when the photosensitive group is restricted by the main chain, the group is difficult to rapidly react with the polarized light radiated to the alignment film, and thus liquid crystal alignment or alignment speed is deteriorated. The deterioration in liquid crystal alignment or alignment speed reduces the process efficiency. Further, the alignment of the liquid crystals is insufficient in the manufactured liquid crystal display, and thus there are problems in that a dichroic ratio becomes small and contrast degradation may occur.